/*
 * Copyright (C) 2016 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import java.util.ArrayList;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Optional;
import java.util.stream.Collector;
import javax.annotation.Nullable;

/**
 * Collectors not present in {@code java.util.stream.Collectors} that are not otherwise associated
 * with a {@code com.google.common} type.
 *
 * @author Louis Wasserman
 * @since 21.0
 */
@Beta
@GwtCompatible
public final class MoreCollectors {

    /*
     * TODO(lowasser): figure out if we can convert this to a concurrent AtomicReference-based
     * collector without breaking j2cl?
     */
    private static final Collector<Object, ?, Optional<Object>> TO_OPTIONAL =
            Collector.of(ToOptionalState::new, ToOptionalState::add, ToOptionalState::combine,
                    ToOptionalState::getOptional, Collector.Characteristics.UNORDERED);

    /**
     * A collector that converts a stream of zero or one elements to an {@code Optional}. The
     * returned collector throws an {@code IllegalArgumentException} if the stream consists of two
     * or more elements, and a {@code NullPointerException} if the stream consists of exactly one
     * element, which is null.
     */
    @SuppressWarnings("unchecked")
    public static <T> Collector<T, ?, Optional<T>> toOptional() {
        return (Collector) TO_OPTIONAL;
    }

    private static final Object NULL_PLACEHOLDER = new Object();

    private static final Collector<Object, ?, Object> ONLY_ELEMENT = Collector.of(ToOptionalState::new,
            (state, o) -> state.add((o == null) ? NULL_PLACEHOLDER : o), ToOptionalState::combine, state -> {
                Object result = state.getElement();
                return (result == NULL_PLACEHOLDER) ? null : result;
            }, Collector.Characteristics.UNORDERED);

    /**
     * A collector that takes a stream containing exactly one element and returns that element. The
     * returned collector throws an {@code IllegalArgumentException} if the stream consists of two
     * or more elements, and a {@code NoSuchElementException} if the stream is empty.
     */
    @SuppressWarnings("unchecked")
    public static <T> Collector<T, ?, T> onlyElement() {
        return (Collector) ONLY_ELEMENT;
    }

    /**
     * This atrocity is here to let us report several of the elements in the stream if there were
     * more than one, not just two.
     */
    private static final class ToOptionalState {
        static final int MAX_EXTRAS = 4;

        @Nullable
        Object element;
        @Nullable
        List<Object> extras;

        ToOptionalState() {
            element = null;
            extras = null;
        }

        IllegalArgumentException multiples(boolean overflow) {
            StringBuilder sb = new StringBuilder().append("expected one element but was: <").append(element);
            for (Object o : extras) {
                sb.append(", ").append(o);
            }
            if (overflow) {
                sb.append(", ...");
            }
            sb.append('>');
            throw new IllegalArgumentException(sb.toString());
        }

        void add(Object o) {
            checkNotNull(o);
            if (element == null) {
                this.element = o;
            } else if (extras == null) {
                extras = new ArrayList<>(MAX_EXTRAS);
                extras.add(o);
            } else if (extras.size() < MAX_EXTRAS) {
                extras.add(o);
            } else {
                throw multiples(true);
            }
        }

        ToOptionalState combine(ToOptionalState other) {
            if (element == null) {
                return other;
            } else if (other.element == null) {
                return this;
            } else {
                if (extras == null) {
                    extras = new ArrayList<>();
                }
                extras.add(other.element);
                if (other.extras != null) {
                    this.extras.addAll(other.extras);
                }
                if (extras.size() > MAX_EXTRAS) {
                    extras.subList(MAX_EXTRAS, extras.size()).clear();
                    throw multiples(true);
                }
                return this;
            }
        }

        Optional<Object> getOptional() {
            if (extras == null) {
                return Optional.ofNullable(element);
            } else {
                throw multiples(false);
            }
        }

        Object getElement() {
            if (element == null) {
                throw new NoSuchElementException();
            } else if (extras == null) {
                return element;
            } else {
                throw multiples(false);
            }
        }
    }

    private MoreCollectors() {}
}
